1. Field of the Invention
This invention relates to a continuous annealing method and a continuous annealing apparatus for cold rolled steel strips, and more particularly to a method and an apparatus for continuously annealing steel strips in a manner effectively preventing meandering and heat buckling of the strips during heat-treatment.
2. Description of the Prior Art
In general, continuous annealing furnaces for cold rolled steel strips are of a vertical type in view of the cost and area for providing the furnaces. In such vertical continuous annealing furnaces, a number of hearth rolls 1 are arranged in upper and lower portions of the furnace 2. A steel strip S is trained around these upper and lower hearth rolls alternately upward and downward in a serpentine path during which these steel strips S are subjected to predetermined heat-treatment required to obtain their material characteristics as shown in FIG. 1.
In continuously heat-treating steel strips in furnaces in such a manner, however, the steel strips often undergo meandering to obstruct smooth operation probably owing to particular shapes of the steel strips or unbalance in tensile force in the steel stripe or particular temperature conditions in the furnaces In order to prevent the meandering, crown or tapered rolls 1' or 1" as hearth rolls have been usually used, which include tapered ends as shown in FIGS. 2a and 2b, which cause centering forces urging the steel strip toward maximum diameter portions of the rolls at their center to prevent the meandering of the strip. However, the centering force tends to exceed a certain level to cause buckling of the strip in its width direction resulting in defects of the steel products, called "heat buckling".
In order to prevent the meandering and heat buckling in this case, therefore, it is required to provide crowning or tapered amount on the rolls so as not to cause the meandering and heat buckling of the strips. However, it is very difficult to determine the crowning or tapered amount because they are caused by various parameters. For example, the heat buckling will increase, the higher the temperature of the heat-treatment, the wider and thinner the steel strips, and higher the feeding speeds of the strips in the furnaces.
Hearth rolls capable of changing their crowning or tapered amounts have been proposed to solve the above problems as disclosed in Japanese Laid-open Utility Model Application No. 55-172,359, Japanese Laid-open Patent Application No. 57-177,930 and Japanese Laid-open Patent Application No. 58-105,464. In order to control the crowning or tapered amounts, however, it is required to provide measuring devices for measuring crowning amounts at every hearth rolls and control devices for controlling the crowning amounts on the basis of the measured amounts in the measuring devices. Such systems, therefore, are very expensive and include a problem of low responsibility to be solved.
Steel strips of carbon content less than 0.1% are generally used for deep drawing. As the melting technique improves, extremely low-carbon steels including carbon content of the order of less than 0.005% have been used for materials for deep drawing. These cold rolled steel plates for deep drawing are to be annealed at temperatures higher than 800.degree. C. and tend to cause the heat buckling. Such a tendency is more acute in low-carbon steels as the carbon content becomes extremely low.
Recently, needs of very thin steel strips having thicknesses less than 0.2 mm increase for blank materials of tin plates. Such very thin steel strips tend to cause the heat buckling as the speed for feeding the strips through furnaces is increased. Moreover, blank materials of extremely low-carbon steels for soft tin plates often cause the problem of heat buckling.